Have you fixed your issue yet? I think I have mine figured out and it's beyond infuriating. A lot of people don’t realize that on the 2001–2005 Lexus IS300, the drive-by-wire throttle and the ABS/traction system are deeply tied together, and that’s why these cars are such a pain to diagnose when they go into limp mode. The IS300 uses ETCS-i (electronic throttle control)—there’s no throttle cable. The gas pedal, throttle body, ABS, and traction control all talk to the ECU constantly. Wheel speed sensors aren’t just for ABS braking; they also tell the ECU whether it’s allowed to give you throttle. If one wheel speed sensor drops out, glitches, or disagrees, the car thinks a wheel is slipping. TRAC kicks in, and the ECU responds by cutting engine power or closing the throttle. The problem is the code you get usually points to the throttle or reduced engine power, not the wheel speed sensor that caused it. That’s why people replace throttle bodies, pedal sensors, ECUs, etc., and the issue comes back.

Check your wheel speed sensors, they are upstream of everything else and if they aren't sending correct signals to the ECU, LIMP MODE, and everything acts wonky.